The present invention is related to a device which yields an electrical output signal but has an input or intermediate signal of the thermal type. Such a device can be used to characterize chemical and physical processes which are accompanied by changes in heat content or enthalpy. Furthermore a method is disclosed for manufacturing said device by means of micromachining which is a technique closely related to the technique used for the manufacturing of integrated circuits.
New approaches in the combinatorial chemistry have resulted in the capability of producing millions of compounds in a short time. Analysis of each compound with respect to multiple parameters is proving to be a significant bottleneck as in e.g. M. A. Shoffner et al., Nucleic Acids Research, 1996, vol. 24, No. 2, pp. 375-9. The number of cells, the test reagent volumes, the throughput rate and the ease of use through automation are all important parameters which should be optimized in order to meet the stringent requirements for modern drug screening. Furthermore a small amount of precious reagent reduces both cost and waste, and increases the number of possible analyses. A candidate for this kind of analysis is a calorimeter. A calorimeter is a device which yields an electrical output signal but has an input or intermediate signal of the thermal type. Calorimetry, more than pH-metry, offers the advantage of generality: all chemical and physical processes are accompanied by changes in heat content, or enthalpy. In fact microcalorimeters can be used for the analysis of the activity of biological cells, chemical reactions in small volumes and other microanalytical applications.
The most frequently used commercially available calorimeters are the Thermometric 2277 Thermal Activity Monitor and the MicroCal MCS Isothermal Titration Calorimeter. They are both based on the use of two or more thermoelectric devices, so called thermopiles, having a common heat sink as reference. A thermopile is at least one thermocouple which is a temperature sensing element and which is connected to identical thermocouples in parallel thermally and in series electrically. Thermocouples do not measure the temperature itself, but rather the temperature difference between two junctions. An advantage of using thermocouples as temperature sensing elements is that there is no offset, i.e. when there is no temperature difference there is no voltage, which makes calibration superfluous. A thermocouple as illustrated in FIG. 2, i.e. a combination of two different (semi)conductive materials, converts a thermal difference between its two junctions into a voltage difference by means of the combined Seebeck coefficient S of its two structural thermo-electric materials. In fact a thermocouple comprises a first conductive material (14) and a second conductive material (13) with an insulating layer (15) inbetween. A thermocouple has a so-called hot junction (11), where said first material and said second material are short-circuited, and a so-called cold junction (15), where said first and said second material are separated one from another by means of said insulating layer. At said cold junction the electrical output signal, representing the temperature difference xcex94T between said hot junction and said cold junction, can be measured.
The total generated voltage is the sum of the thermocouple voltages. For n (n being a positive whole number greater than zero) thermocouples, where each thermocouple is identical, it can be written that:
Utp=n*S*xcex94xcex94T.
The temperature difference xcex94T is the product of the generated power difference between the two junction sites and the thermal resistance:
xcex94T=xcex94Pgen*Rth
Thermopiles are preferred because they are self-generating, easy to integrate and because the temperature changes involved are low frequency signals.
The drawbacks of these state-of-the art devices are the following. These devices have at least two thermopiles and a common heat sink. The cold junctions of each thermopile are thermally coupled to the common heat sink which is at a known temperature. The hot junctions of each thermopile are thermally coupled to a substance under test. So in fact, one tries to perform a kind of absolute measurement by measuring the temperature difference between this substance under test and the heat sink at known temperature. By applying different substances under test to different thermopiles as e.g. for drug screening where the hot junctions of a first thermopile are coupled to reference cells and the hot junctions of a second thermopile are coupled to genetically engineered cells expressing a drug target. When the potential drug candidate is effective, it will activate the genetically engineered cells which results in a heat change. This heat change is determined indirectly by subtracting the measured signals of the first and the second thermopile, where the cold junctions of both thermopiles are coupled to a common heat sink at known temperature. This is a cumbersome approach which lacks accuracy and demands a space consuming design.
In an aspect of the invention a device is disclosed based on only one thermopile wherein said thermopile is in contact with at least parts of a substrate, e.g. a silicon wafer or the remains thereof. The cold junctions of said thermopile are coupled thermally to a first channel comprising a first substance while the hot junctions of said thermopile are coupled thermally to a second channel comprising a second substance, said first and said second channel are separated and thermally isolated one from another. Said device is capable of handling a very small amount of a substance, typically in the range from 1 microliter to 30 microliter.
In an aspect of the invention a device for monitoring chemical and physical processes which are accompanied by changes in heat content or enthalpy is disclosed, comprising a thermopile, wherein said thermopile is in contact with at least parts of a substrate, e.g. a silicon wafer or the remains thereof, and wherein said thermopile is a set of at least one thermocouple comprising a first conductive material and a second conductive material with an insulating layer inbetween. Said first and said second material are chosen such that their thermo-electric voltages are different. A first substance, i.e. a reference substance, can be thermally coupled to the cold junctions of said thermopile while a second substance, i.e. a test substance, can be thermally coupled to the hot junctions of the same thermopile. Alternatively, a first substance, i.e. a test substance, can be thermally coupled to the cold junctions of said thermopile while a second substance, i.e. a reference substance, can be thermally coupled to the hot junctions of the same thermopile. To speed up measurement time or to test a number of substances at the same time, a modular system comprising an array of devices, each device comprising one thermopile, can be configured on the same substrate. Said device can further comprise a thin insulating layer, e.g. an oxide layer or a nitride layer, on said thermopile in order to prevent a direct contact between the substances and the thermopile to thereby avoid damaging said thermopile. Said device further comprises a membrane to thermally and electrically isolate said thermopile and to mechanically support said thermopile. Silicon oxide and/or silicon nitride can be used as membrane materials. Particularly a liquid rubber, i.e. ELASTOSIL LR3003/10A, B can be used as a membrane material.
In an aspect of the invention a method is disclosed for fabricating a device used to monitor chemical and physical processes which are accompanied by changes in heat content or enthalpy. The device is capable of handling a very small amount of a substance. These requirements can be achieved by micromachining, a technique closely related to integrated circuit fabrication technology. The starting material is a substrate like e.g. a semiconductive wafer, particularly a monocrystalline silicon wafer, or a slice of an insulating material, i.e. a glass slice. On this substrate layers can be coated, patterned by means of a sequence of lithographic steps and wet and/or dry etching steps. Such processed substrates can be bonded to each other or to other materials in order to make three-dimensional structures.
According to the invention a method is disclosed for fabricating rubber membranes. This method comprises the following steps:
On a first side of a substrate a silicon oxide/silicon nitride stack is deposited which will serve as an etch mask to define the membrane pattern. Other materials and/or other thickness and or another number of layers may be used to serve as an etch mask. Said substrate can be a semiconductive wafer or slice, like e.g. a silicon wafer, or an insulating slice like e.g. a glass slice.
On the second side of the substrate a silicon oxide/silicon nitride stack is deposited which will serve as an etch stop to define the membrane pattern. Other, preferably insulating, materials and/or other thickness and or another number of layers may be used to serve as an etch stop. One can also choose to omit this etch stop dependent on the etch procedure.
The etch mask on said first side of the substrate is patterned by means of a sequence of photolithographic steps and wet and/or dry etching steps.
The second side of the substrate is coated with liquid rubber, i.e. ELASTOSIL LR3003/10A, B. The relatively low viscosity of said rubber allows for a spin-coating technique. The surface of the substrate is chemically modified to make it water repellent by treating said surface with hexamethyldisiloxane (HMDS).
A second substrate is bonded onto the first by means of the unvulcanised rubber. The bonding is performed in low vacuum and the rubber is cured. Alternatively, instead of a second wafer a glass plate is used. A first side of this glass plate comprises a wax layer to protect the rubber layer of the first substrate because said first side is exposed to the rubber during the bonding process.
To free the membrane a chemical back etch is performed in KOH.
In an aspect of the invention a method is disclosed for fabricating a device used to monitor chemical and physical processes which are accompanied by changes in heat content or enthalpy. This method or process comprises the following steps:
On a first side of a substrate at least one hard mask layer is deposited which will serve as an etch mask for removing at least parts of said substrate.
On the second side of the substrate at least one hard mask layer can be deposited which can serve as an etch stop layer dependent on the etch procedure used and/or as an insulating layer to thermally and electrically isolate a thermopile and/or to inhibit a direct contact between a substance and said thermopile.
On said second side of the substrate a first conductive layer with a thickness typically in the range from 0.3 xcexcm to 1 xcexcm is deposited. Said first conductive layer, e.g. a doped polysilicon layer, is patterned to thereby form the first material of the thermopile, i.e. a set of thermocouples which are connected in parallel thermally and in series electrically.
On said first side of the substrate said hard mask layer is patterned in order to define the etch windows for etching away the underlying silicon in order to expose at least parts of the underlying thermopile or the etch stop multi-layer structure on said thermopile.
On said second side of the substrate an insulating layer is deposited with a thickness in the range typically from 0.2 xcexcm to 1 xcexcm or from 0.5 xcexcm to 5 xcexcm. Said insulating layer is used as an interconductive layer dielectric and isolates the different fingers of the pattern of the first conductive layer from each other. Said insulating layer is patterned to thereby form via holes through which the underlying first conductive layer can be contacted in order to form hot junctions.
A second conductive layer, having a thermo-electric voltage different from the thermoelectric voltage of said first conductive layer, i.e. an aluminum layer with a thickness of 200 nm, is deposited on said second side of the substrate. Said second conductive layer is patterned to thereby form the second material of the thermopile.
On said second side of the substrate an insulating layer is deposited to serve as a membrane. Said membrane should thermally and electrically isolate said thermopile and mechanically support said thermopile. Silicon oxide and/or silicon nitride can be used as membrane materials, but preferably a liquid rubber, i.e. ELASTOSIL LR3003/10A, B is used.
A second substrate can be bonded or glued onto said first substrate.
To expose the thermopile and the membrane, a chemical back etch is performed. Said back etch can be a sequence of etching steps using KOH as an etchant and wherein each etching step is performed with a weight percentage of KOH in the range from 20 to 60 percent and at a temperature in the range from 20 to 100xc2x0 C. Alternatively an etch mask, e.g. a silicon oxide/silicon nitride stack, can be deposited and patterned on the free surface of said second substrate. By doing so, during the back etch e.g. only the glass or silicon underneath the thermopile is removed to thereby free the membrane.